1. Field of the Invention
The present invention generally relates to optical scanning devices, and more particularly to a novel device and technique for reading bar codes on a transparent substrate using solid state components.
2. Description of the Prior Art
The present invention is directed to certain improvements on the Image Access System (IAS) electronic presentation system for displaying stored images, as disclosed in U.S. Pat. Nos. 4,994,987, 5,001,696 and 5,233,171, and in U.S. patent application Ser. No. 07/861,067. The terms "Image Access System" and "IAS" are trademarks of Minnesota Mining and Manufacturing Co. (3M--assignee of the present invention).
The IAS presentation system requires a substrate, such as a preview card, which contains human-readable information (an illustration or textual description) associated with a stored image, and machine-readable information associated with the location of the stored image, e.g., a software address on a medium separate from the card, such as a computer diskette. One embodiment of the IAS system includes a card reader which supports the card in such manner that it may be viewed by the user and simultaneously scanned by the reader to allow decryption of the machine-readable information. In each of the three embodiments of the preview cards described in U.S. Pat. No. 5,001,696, the machine-readable information (bar code) has an opaque background (the white paper card) and is designed to be read using a reflective (or retroreflective) scanning technique. For example, in U.S. Pat. No. 5,233,171, the card reader uses the same optical path (inside the reader) for emitting the light beam and for collecting the reflected signals. See also U.S. Pat. Nos. 4,644,143 and 5,053,612 and European Patent Application No. 550,271, which show bar code readers requiring complicated constructions using beam splitters, mirrors and lenses to create folded optical paths. Hand-held scanners having similar folded optical paths are disclosed in U.S. Pat. Nos. 5,184,005, 5,192,856, 5,210,398 and 5,216,523. Reference is also made to the publications cited in the above-mentioned IAS patents and application.
The use of retroreflective collection techniques, in particular, imparts several advantages, such as minimizing reception of erroneous specular reflections. Unfortunately, however, the use of reflective (and retroreflective) scanning techniques or a folded optical path generally requires more space in the reader, detracting from the desideratum of compactness, which enhances the reader's portability. Reflective techniques also require a relatively high intensity (radiance) light source (e.g., a laser or laser diodes) since there is never 100% reflection of the beam and some scattering always occurs. It would, therefore, be desirable to devise a bar code reader which uses transmissive collections techniques, rather than reflective, in order to simplify the optical geometry of the reader and eliminate the need for high-intensity light sources. It would be further advantageous if the reader included a two-way communications protocol to facilitate the transmission of the optically encoded data to a host system, such as a computer which displays the electronically stored images associated with the optically encoded data, and to provide feedback from the host computer with respect to the current presentation status.